He is the son of Forrest Mars, Sr., and grandson of Frank C. Mars, founders of the Americancandy company Mars, Incorporated. As a member of the Mars family, his share of the company and other assets were worth US$10 billion in September 2010,[4] making him the 52nd richest person in the world and the 26th richest person in America, according to Forbes.[4] As of October 2012, Mars is listed as the 30th richest person in the world with an estimated net worth of 20.1 billion.[5] According to Hurun Global Rich List 2015 he is 28th richest person in the world with a net worth of US$26 billion.

He married Adrienne Bevis in June 1958.[2][6] They have three children: Linda Anne Mars born May 23, 1960 (1960-05-23) (age 54), Frank Edward Mars born March 13, 1963 (1963-03-13) (age 51), and Michael John Mars born November 22, 1967 (1967-11-22) (age 47).[2][3]